# Identifying three identical metal bars (with your eyes closed )

1. Feb 21, 2009

### IRNB

Identifying three identical metal bars (with your eyes closed!!)

1. The problem statement, all variables and given/known data

How would you determine, non-destructively, and with eyes closed, which is which amongst three identically-shaped metal bars known to consist of a permanent magnet, a piece of soft iron and a piece of copper?

2. Relevant equations

N/A

3. The attempt at a solution

I know you could identify which is the copper bar because it wouldnt be attracted to either of the other two bars but im not sure how to determine which bar has the permanent magnet and which has the soft iron.

Could you perhaps use the effect of eddy currents to determine which is the parmanent magnet and which is the bar with the piece of soft iron. What i'm thinking is, trying to move the copper bar in the presence of the parmenant magnet should be more difficult then moving it in the presence of the soft iron bar.

2. Feb 21, 2009

### Delphi51

Re: Identifying three identical metal bars (with your eyes closed!!)

Well, you beat me to it! Outstanding work!

3. Feb 21, 2009

### Chi Meson

Re: Identifying three identical metal bars (with your eyes closed!!)

I don't think the eddy currents would be strong enough to realistically tell the difference, but theoretically, it should work.

There is a more realistic method of determining which is the soft iron and which is the magnet. It is something that anyone can do and see. IT has to do with one picking up the other. Putting the bars end to end wouldn't make the distinction, but what's an alternative way of one picking up the other?